This invention pertains to lubricants and, more particularly, to a grease particularly effective for lubrication of automotive bearings.
Lubrication of wheel bearings has been practiced almost since bearings were used to promote efficient rotational motion of wheels. In older cars, wheel bearings were periodically removed, cleaned to remove old grease and any contaminants, and repacked with new grease. The performance requirements of such automotive wheel bearing greases were much less since re-lubrication occurred at regular intervals.
In recent years however, there has been a switch to sealed-for-life automotive wheel bearings. The grease used in these bearings must provide all the lubrication requirements for the entire life of the bearing. Since most automotive manufacturers want these wheel bearings to last for the entire life of the car, this places an enormously increased demand on the grease. Also, recent changes in bearing and drive train design have further increased grease performance requirements by sometimes increasing the internal loading of the bearings beyond the purely hydrodynamic or elastohydrodynamic regime previously experienced. The result of these changes is that wheel bearing grease used in older vehicles may not be satisfactory for outstanding performance in today's vehicles. Also, many automotive manufacturers want a wheel bearing grease which will also perform well in other automotive bearing applications such as alternator bearings, water pump bearings, and air conditioner compressor bearings.
Greases which offer truly outstanding performance in today's automotive wheel bearings must simultaneously meet numerous performance criteria. The most important property needed by a high performance automotive wheel bearing grease is long bearing life. The grease must protect the bearings for long periods of time at sustained temperatures which can reach 350.degree. F. or higher. Perhaps the best measure of this performance attribute is the high temperature bearing test, ASTM D3336, especially when run at 350.degree. F. ASTM D3336, bearing life at 350.degree. F. has been mostly limited to 600 hours to 800 hours in prior art greases. Superior performance is sought. Specifically, ASTM D3336 bearing lives at 350.degree. F. of at least 1,000 hours are desired to assure outstanding performance.
To obtain such improved ASTM D3336 performance requires improvements in many other performance-related properties. The grease must exhibit a high dropping point, at least 450.degree. F. The grease must exhibit reduced oil separation, especially at high temperatures such as 300.degree. F. to 350.degree. F. Excellent oxidation and thermal stability is needed. A minor amount of extreme pressure (EP) and antiwear (AW) performance is needed, especially to reflect some of the more modern design changes in today's automotive wheel bearings. This requirement is, however, belied by the fact that traditional EP/AW additives are extremely deleterious to high temperature bearing life. For instance, inclusion of organo-sulfur EP/AW additives are well known to reduce ASTM D3336 bearing life at 350.degree. F. by 80% or more, even when such additives are present in small to moderate levels. While this phenomenon is not well understood, one theory is that the traditional EP/AW additives accelerate corrosive fatigue of the bearings due to their well known high temperature corrosivity. Therefore, in a high performance automotive wheel bearing grease, any EP/AW properties must be provided while maintaining excellent non-corrosivity at high temperatures.
Besides those properties contributing to excellent high temperature bearing life, other properties required by high performance automotive wheel bearing greases include excellent corrosion (rust) protection, even in the presence of salt water.
A high performance automotive wheel bearing grease should also provide excellent fretting wear protection at low temperature. This property stems from the shipment of finished cars by truck over cold mountainous terrain. Under such transport, the wheels will "jiggle" for many hours. This oscillatory motion is further complicated by the low temperatures which can be experienced. Prior art wheel bearing greases have been used which provided less than adequate protection against such conditions. The result was cars arriving at their shipping destination with high levels of fretting wear in he wheel bearings.
Yet another property required by high performance automotive wheel bearing greases is minimal high temperature outgassing. The grease should not generate large quantities of gaseous products when held at high temperatures. This is because the wheel bearing is sealed by an elastomer to minimize environmental contamination. Any gaseous products given off by the grease at high temperatures will put a stress on the seal and in extreme cases cause the seal to pop or break. Some automotive bearing seals have vent holes to prevent this from occurring, but unfortunately not all seals have this design safeguard. One large American automotive manufacturer has specified that wheel bearing greases must not produce in excess of 28 pounds per square inch (psi) of outgassing pressure at 350.degree. F. and no more than 5 psi after the grease has cooled to 75.degree. F. The test method used in this determination is described in a subsequent example.
Yet another desired property of high performance automotive wheel bearing greases is that they contribute minimally to bearing noise during bearing operation. Such greases are often referred to as quiet greases. Surprisingly, it has been found that all greases contribute to noise during bearing operation. However, not all greases contribute equally to the noise. The reason that the acoustic properties of a wheel bearing grease are important has to do with bearing manufacturing quality control. One effective, efficient, and economical way to determine if newly manufactured bearings have manufacturing flaws is to determine their acoustical properties during use. If the grease in them is too noisy, it may mask the characteristic acoustical properties which would otherwise tell the quality control technician whether the bearing is or is not flawed. A grease will tend to be more quiet if it possesses a smooth texture. Experience with two recent wheel bearing greases indicated that the one which was significantly quieter also possessed an extremely smooth texture and a semi-translucent, glassy appearance. The other wheel bearing grease had a less smooth texture and an opaque, waxy appearance.
Over the years, a variety of lubricants have been used and suggested for use to lubricate automotive wheel bearings. These compositions have met with varying degrees of success, since they are usually deficient in one or more of the above categories and do not possess all the above mentioned desirable qualities.
It is therefore, desirable to provide an improved automotive wheel bearing grease which overcomes many, if not all, of the preceding problems.